It is conventional to investigate the characteristics of oil contained in an underground formation by taking samples and raising them to the surface. However, in order to ensure that oil taken in this way from a well retains the properties which it has at a distance from the well in said formation, it is necessary for the oil to be in a single phase condition at the point where a sample is taken. Otherwise, the samples obtained are not representative of the bulk of the oil in the formation.
Generally, the single phase state of the oil being sampled in a well is verified by measuring the bubble point of sample raised to the surface, i.e. by measuring the pressure above which the gases mixed with the oil are completely dissolved in the oil so that the oil behaves as a single phase liquid. Given the pressure and the temperature down the well where the sample is taken, it is possible to deduce whether the oil at that location is indeed in a single phase state. Performing such a bubble point measurement in the laboratory requires the oil to be re-compressed to a pressure of the same order as the down-hole pressure, and also requires the oil to be heated to the temperature of the underground formation.
This type of measurement therefore requires lengthy operations and the results indicating whether or not the samples obtained are valid are obtained only a considerable length of time after the samples were taken, and in some cases that may mean that it is already too late to take new samples.
Preferred implementations of the present invention avoid the above drawbacks by performing bubble point measurements rapidly and down-hole, thereby enabling the validity or otherwise of samples being taken to be established on site and in good time.